Devices for the storage of electrical energy can be described as batteries or capacitors based upon the mechanisms used for energy storage and the discharge characteristics they exhibit. For example, see B. E. Conway, J. Electrochem. Soc., v138, n6, pp. 1539-48, 1991. In rechargeable batteries, energy is stored almost entirely through reversible oxidation-reduction (redox) reactions. Although a double layer exists at the electrode surfaces, it contributes very little to the stored energy. In capacitors, a significant amount of energy is stored in the double layer at the electrode surfaces although in some cases, as described by Conway above, a major fraction of stored energy is attributable to reversible surface redox reactions.
As compared to batteries, electrochemical capacitors can be characterized as having low energy density, high power density and a high cycle life as described in S. Sarangapani et. al., Journal of Power Sources, v29, pp. 355-64, 1990. Further, in an electric circuit, an electrochemical capacitor behaves more like a classical dielectric capacitor than a battery, hence its name.
The fundamental unit or cell of an electrochemical capacitor consists of two electrodes and an electrolyte. Both double layer and redox reactions may occur at one or both electrode surfaces. The energy storage of the device is limited by the decomposition voltage of the electrolyte and the available electrode surface area. The decomposition voltage is the voltage at which non-reversible redox reactions occur between the electrode surface and the electrolyte. For example, capacitors utilizing an aqueous electrolyte are limited to the potential at which water electrolyzes (about 1.2 volts). A capacitor employing a solid electrolyte such as RbAg.sub.4 I.sub.5 is limited to a cell voltage of less than 0.7 volts.
The conventional method for making solid electrolyte capacitors is based upon the use of powdered materials for the electrolyte that are pressed or held together with a binder. U.S. Pat. Nos. 3,419,760 and 5,047,899 exemplify the pressed powder method for solid electrolyte capacitors.
One way to increase the voltage of electrochemical cells is to stack a number of cells together in series, similar to a multi-cell battery. However, multi-cell electrochemical capacitors have been limited in voltage due to the difficulty of manufacturing devices containing a large numbers of cells with sufficiently low equivalent series resistance. These devices have also been limited in application due to their relatively high cost of production. Another problem in producing electrochemical capacitors is that virtually all of the prior art methods for assembling a multi-cell capacitor involve stacking or aligning a series of individual electrodes and subsequently packaging the assembled electrodes as a single capacitor. This method of assembly requires the handling of a large number of elements with close tolerances for each device. For devices with electrodes and standoffs with small dimensions, the alignment becomes very important with respect to short circuits between cells in a device. In the case of solid electrolyte devices, multi-cell stacks are seldom used due to the difficulty of working with powders. Also, the minimum electrode thickness that can be achieved limits the number of cells that can be assembled in a practical device.
As stated above, the energy density of electrochemical capacitors is inferior to that of batteries. This is in part due to the volume of the electrode and electrolyte that does not participate in the storage of energy. Since energy storage occurs at the interfacial surface, the interior volume of the electrode and electrolyte layers is essentially wasted. In addition, the interior volume of the electrode and electrolyte layers contributes to equivalent series resistance of the device, with the bulk being due to the electrolyte. Prior art methods which utilize powders have not been effective in minimizing the interior volume of the electrolyte between the two electrodes. Thus, high conductivity electrolytes such as sulfuric acid are used in spite of the difficulties associated with the containment of a corrosive liquid. The use of solid electrolytes offers the advantage of a more stable and more reliable device at the expense of greater electrolyte resistivity and a typically lower decomposition voltage than aqueous electrolytes such as sulfuric acid. However, the prior art has not effectively reduced the electrolyte thickness. Decreasing the thickness of the solid electrolyte layer would decrease the resistance of the layer and compensate for the higher resistivity of the solid electrolyte. A decreased electrolyte thickness would also permit more cells to be stacked in a device of a given height, thereby compensating for the relatively low decomposition voltage of the solid electrolyte.
Accordingly, it is an object of the invention to provide a method for making a bipolar electrode suitable for use in an electrochemical capacitor. It is another object of the invention to provide an improved method of manufacturing electrochemical capacitors having a thinner solid electrolyte layer. It is a further object of the invention to provide a method of producing multiple cell electrochemical capacitors with a high energy density, high working voltage and a low equivalent series resistance.